1. Field of the Invention
The present invention is directed generally to gas sensors and, more specifically, to gas sensors having a sensor element whose electrical resistance or conductivity depends upon the partial pressure of the gas being detected.
2. Introduction
Exhaust gas sensors are generally exposed to a gas mixture containing several reactive constituents. If the gas-sensitive element consists of a metal oxide, the reversible reciprocal effects that standardly occur at high temperatures (volume reactions, adsorption and desorption processes) of the sensor material with the target gas are exploited in order to measure the concentration or, respectively, partial pressure of this gas. However, the metal oxide often also continues to interact with other constituents of the gas mixture. These can in particular be chemical reactions that can finally lead to the destruction of the sensor layer, which is only a few .mu.m thick, or, respectively, can irreversibly alter its characteristics. In order to ensure the required long service life and high reliability of the gas sensors, it is absolutely necessary to avoid such reactions. This problem can for example be solved by covering the gas-sensitive sensor regions with a porous protective layer whose material bonds chemically with the materials that damage the metal oxide.
The oxygen sensor of a rapid .lambda. probe, known from DE 4339737 C1 and shown in cross-section in FIG. 1, consists essentially of the two comb electrodes 2/2', arranged on an Al.sub.2 O.sub.3 substrate 1, the oxygen-sensitive SrTiO.sub.3 layer 3, and a porous SrTiO.sub.3 protective layer 4. The protective layer 4, which completely covers the oxygen-sensitive sensor regions, is exposed to the exhaust gas of an internal combustion engine. Besides nitrogen oxides (NO.sub.x), carbon monoxide (CO) and hydrocarbons (CH.sub.x), the exhaust gas of a typical internal combustion engine also contains, among others, SiO.sub.2, MnO.sub.2, Fe.sub.2 O.sub.3, P.sub.2 O.sub.5, Cl.sub.2, and SO.sub.2, due to abrasion and the additives added to the fuel or, respectively, motor oil. The gaseous compounds react with the strontium (Sr) and the titanium (Ti) of the protective layer 4, e.g. to form TiO.sub.2, Sr.sub.3 (PO.sub.6).sub.2, TiCl.sub.4, and thus do not reach the sensitive layer 3. Moreover, the protective layer 4 catches the particles of SiO.sub.2, MnO.sub.2 and Fe.sub.2 O.sub.3. The SrTiO.sub.3 protective layer 4 considerably prolongs the service life of the known oxygen sensor. However, the observed drift of the sensor signal due to the alteration of the protective layer is disadvantageous.